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Unformatted text preview: 5 Thermochemistry Visualizing Concepts 5.2 (a) The internal energy, E, of the products is greater than that of the reactants, so the diagram represents an increase in the internal energy of the system. (b) ∆ E for this process is positive, +. (c) If no work is associated with the process, it is endothermic. 5.3 (a) For an endothermic process, the sign of q is positive; the system gains heat. This is true only for system (iii). (b) In order for ∆ E to be less than 0, there is a net transfer of heat or work from the system to the surroundings. The magnitude of the quantity leaving the system is greater than the magnitude of the quantity entering the system. In system (i), the magnitude of the heat leaving the system is less than the magnitude of the work done on the system. In system (iii), the magnitude of the work done by the system is less than the magnitude of the heat entering the system. None of the systems has ∆ E < 0. (c) In order for ∆ E to be greater than 0, there is a net transfer of work or heat to the system from the surroundings. In system (i), the magnitude of the work done on the system is greater than the magnitude of the heat leaving the system. In system (ii), work is done on the system with no change in heat. In system (iii) the magnitude of the heat gained by the system is greater than the magnitude of the work done on the surroundings. ∆ E > 0 for all three systems. 5.5 w = –P ∆ V. Since ∆ V for the process is (–), the sign of w is (+). ∆ E = q + w. At constant pressure, ∆ H = q. If the reaction is endothermic, the signs of ∆ H and q are (+). The sign of w is (+), so the sign of ∆ E is (+). The internal energy of the system increases during the change. (This situation is described by the diagram (ii) in Exercise 5.3.) 5.7 (a) N 2 (g) + O 2 (g) → 2NO(g). Since ∆ V = 0, w = 0. (b) ∆ H = 90.37 kJ for production of 1 mol of NO(g). The definition of a formation reaction is one where elements combine to form one mole of a single product. The enthalpy change for such a reaction is the enthalpy of formation. 5.8 (a) ∆ H A = ∆ H B + ∆ H C . The net enthalpy change associated with going from the initial state to the final state does not depend on path. The change can be accomplished via reaction A, or via two successive reactions, B then C, with the same net enthalpy change....
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